1. Field of the Invention
This invention relates to roof bolts and, more particularly, to a roof bolt which is positioned in a bore hole drilled in a rock formation in a mine roof and which is held in place within the bore by both a mechanical anchor and a quick-setting resin.
2. Description of the Prior Art
It is a well-established practice in underground mining work, such as coal mining, tunnel excavation or the like, to reinforce or support the mine roof to prevent rock falls or cave-ins. The most common means presently used to support mine roofs is an elongated bolt or bar which is inserted into the rock formation above the mine roof in a bore hole and which is securely fixed to the bore hole by an anchoring means such as a mechanical anchor, a quick-setting resin which surrounds the end of the bolt within the hole, or both. The roof bolt, placed under the tension, is used to hold a metal support plate in close engagement with the roof.
The roof bolt described in U.S. Patent No. 4,655,645 combines the features of a mechanical anchor and resin bonding but also provides positive and complete mixing of the resin components by an additional mixing mechanism. This arrangement is relatively inexpensive and easy to manufacture, forces the resin upwards along the bolt during mixing, more violently mixes the resin for a shorter mix time, and eliminates the use of a two-position coupling or delay mechanism. While this arrangement functions very well in hard rock-like formations, it has been found to be less effective in soft strata.
It is an object of the present invention to provide a roof bolt arrangement which has all of the advantages of the roof bolt disclosed in U.S. Patent No. 4,655,645, yet which is effective in the soft strata type of mine formation.